The English name of laser is the abbreviation of “light amplification by stimulated emission of radiation”, which means “amplification of light through stimulated radiation”.
Laser, like radio wave and microwave, has wave-particle duality;
However, the generation mechanism of the laser is different from that of ordinary light, which determines that it has better characteristics than ordinary light.
The generation of light is related to the atomic motion state inside the light source.
When the motion state of an atom changes, its internal energy will change accordingly.
An atom has a series of discontinuous E1, E2, E3,…, en and other energy states, which is called the stable state of the atom.
These discontinuous energy values are usually called atomic energy levels.
“Atomic energy level” actually refers to the discontinuous level-by-level form of the energy of electrons in an atom, which is a common attribute of all microscopic particles (atoms, ions, molecules, etc.).
The characteristics of atomic energy can be represented graphically by energy levels.
Fig.1 shows the energy level distribution of the simplest hydrogen atom.
The lowest level is E1, which is called the base level (or ground state), and any other level is called the excited level (or excited state).
The energy value of the ground state is recorded as “0”, which does not mean that the internal energy of the ground state atom is “zero”, but that the transformation of the internal energy of the atom caused by the change of the “orbit” of the electron motion starts from here.
Atoms always keep their energy state at the lowest value, that is, the ground state.
If we want to make these particles produce radiation, we must first transition the particles in the ground state to the high energy level.
This process is called excitation, and the state of the atom after excitation is called the excited state.
Make the atom transition from low energy level to high energy level, which means that its internal energy (or state) has changed.
It is necessary to give the atom certain energy, such as heating, illumination, collision and so on. When the energy hν of the external photon absorbed by the particle is exactly equal to E2-E1, the atom will transition from its low energy level to the state with energy E2 (see Fig.2).
Fig.1 energy level of the hydrogen atom
a) The initial atom is at the low energy level E1
b) The atom absorbs photons and is excited to the high energy level E2
For an excited atom or particle, its higher internal energy makes it in an unstable state.
It always tries to return to the lower energy level by means of radiation transition.
The transition of atoms from high energy level to low energy level without external action is called spontaneous transition.
During the spontaneous transition, there are two ways to release energy:
One is to release heat energy, which is called non radiative transition;
The other is if a photon is emitted during the transition and radiated in the form of light, which is called spontaneous emission transition.
The frequency v of the emitted photon is determined by the energy difference between the two energy levels.
For example, the frequency of the light wave radiated by the transition from energy level E2 to energy level E1.
Where, H – Planck constant.
Its characteristic is that the frequency of each photon in spontaneous emission meets Planck formula Hν= E2-E1；
When the particles on the higher energy level E2 transition, they all emit a photon independently, and these photons are irrelevant to each other.
Therefore, although they have the same frequency, their phase, direction and polarization are different, so they are chaotic, random and uncontrollable.
If the atom in the excited state is induced by external radiation (photons), making the atom in the excited state transition to the low-energy state and emitting a beam of light at the same time, it is called stimulated radiation transition (induced transition).
This beam of light is completely consistent with the incident light in terms of frequency, phase, propagation direction and polarization, which is called stimulated radiation transition.
Stimulated radiation is equivalent to strengthening the external excitation light, that is, it has the function of light amplification.
Therefore, stimulated radiation is the main physical basis of laser generation.
Fig.3 is a schematic diagram of spontaneous emission transition and stimulated emission transition.
Fig.3 schematic diagram of spontaneous emission transition and stimulated emission transition
a) Spontaneous emission transition
b) Stimulated radiation transition
In order to make the stimulated radiation exceed the absorption, the number of particles in the high-energy state must be more than that in the low-energy state, even if the number of atoms in the high-energy level is greater than that in the low-energy level.
This distribution is called particle number inversion.
There are many ways to form particle number inversion.
Generally, the method of gas discharge can be used to excite dielectric atoms with electrons with kinetic energy, which is called electric excitation;
Pulse light source can also be used to irradiate the working medium, which is called optical excitation;
There are thermal excitation, chemical excitation and so on.
Various excitation methods are vividly referred to as pumping or pumping.
In order to continuously obtain the laser output, we must constantly “pump” to maintain that the number of particles at the high energy level is more than that at the low energy level.
The common ones are optical pumping and electric excitation.
Optical pumping is to irradiate and excite the working substance with light, and use the stimulated absorption of the particle system to make the particles of lower energy level transition to higher energy level to form particle number inversion.
For example, the particle number inversion of yttrium aluminum garnet crystal is realized by xenon lamp irradiation, and the electric excitation is to promote the collision between electrons, ions and molecules through the glow discharge of the medium.
And the resonance exchange energy between particles, which makes the particles at the lower energy level transition to the higher energy level to form the particle number inversion, such as the particle number inversion of CO2 gas.
In the stimulated transition, a photon encounters an excited atom and becomes two photons, and the number of photons is doubled.
These two photons can interact with other excited atoms to become four photons.
In this way, the light will become stronger and stronger by reflecting back and forth in the resonant cavity and repeating the above process.
The resonant cavity is composed of multilayer dielectric films directly evaporated on both ends of the working material as mirrors, or two mirrors installed in front of both ends of the working material.
Of the two mirrors, one is totally reflective of the beam and the other is partially permeable.
The light beam is reflected back and forth between the two mirrors to strengthen the excitation, and passes through the working material many times to form oscillation.
The photons along the axial direction interact with the excited particles on the metastable state to generate stimulated radiation, so that the light is further amplified (strengthened), and output as a laser beam at the end equipped with a partial transmission mirror.
For continuous output lasers, the stable laser output is achieved at this time.
For the laser with pulse output, when the laser output is the strongest, the number of high-energy particles decreases and the number of low-energy particles increases due to stimulated radiation, so the next step must be the weakening of the laser until it stops.
Laser is a new light source, which has the characteristics of high directivity, high brightness (photon intensity), high monochromaticity and high coherence.
Because the laser has these characteristics, it is very ideal to use it as processing heat source.
The divergence angle of the laser is very small, close to the parallel light, monochromatic and single frequency.
After focusing by the lens, a very small spot can be formed, and the minimum spot diameter can be equivalent to the order of magnitude of the laser wavelength.
Coupled with the high brightness of the laser, the power density on the focused spot can reach 104 ~ 1015W / cm2 or higher.
Under the irradiation of such high power density light, Converting light energy into heat energy will soon melt or vaporize it.
Therefore, the laser is a good high power density energy for welding, cutting and drilling.